1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving a packet data control channel in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
A cellular mobile communication system is a typical wireless communication system. The mobile communication system uses a multiple access scheme to simultaneously communicate with a plurality of users. A Time Division Multiple Access (TDMA) scheme, a Code Division Multiple Access (CDMA) scheme, and a Frequency Division Multiple Access (FDMA) scheme are typically used as the multiple access schemes. With the rapid progress of technology, the CDMA mobile communication system has developed from a system providing voice communication into a system capable of transmitting high-speed packet data.
An Orthogonal Frequency Division Multiple Access (OFDMA) scheme has recently been proposed to overcome the limit on code resources used in the CDMA scheme.
The OFDM scheme, a scheme for transmitting data using multiple carriers, is a type of a Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbol streams and modulates each of the parallel symbol streams with a plurality of orthogonal sub-carriers, such as sub-carrier channels, before transmission.
A system employing the MCM scheme was first applied to the military High Frequency (HF) radio in the late 1950s, and an Orthogonal Frequency Division Multiplexing (OFDM) scheme for overlapping a plurality of orthogonal sub-carriers. Although the OFDM scheme has developed from the 1970s, it has been limited in its application due to the difficulty in the implementation of orthogonal modulation between multiple carriers. However, since Weinstein et al. showed in 1971 that OFDM modulation/demodulation can be efficiently processed using Discrete Fourier Transform (DFT), the OFDM scheme has rapidly developed. In addition, the advent of a scheme of using a guard interval and inserting a cyclic prefix (CP) symbol in the guard interval reduces the influence of multi-path and delay spread on the system.
Advances in technological developments have led to the popular application of the OFDM scheme to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Television (TDV), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM), among others. That is, the OFDM scheme, which was not popularly used due to its high hardware complexity, can now be implemented with the recent development of various digital signal processing technologies including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT). Although the OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme, it maintains orthogonality between sub-carriers during data transmission. This facilitates optimal transmission efficiency during high-speed data transmission. Since the OFDM scheme has high frequency efficiency and is robust against multi-path fading, it can obtain optimal transmission efficiency during high-speed data transmission. As the OFDM scheme overlaps frequency spectrums, it has high frequency efficiency, and is robust against frequency selective fading and multi-path fading. With the use of a guard interval, the OFDM scheme can reduce inter-symbol interference (ISI), can design an equalizer with a simple hardware structure, and is robust against impulse noise. Therefore, the OFDM scheme tends to be actively applied to the communication system.
In wireless communication, the high-speed, high-quality data service is generally hindered due to the channel environment. The channel environment in the wireless communication undergoes a change due to power variation of a received signal caused by Additive White Gaussian Noise (AWGN) and fading, shadowing, a Doppler effect caused by movement and frequency velocity change of a terminal, and interference caused by other users and a multi-path signal. The above factors must be overcome in order to support the high-speed, high-quality data service in the wireless communication. The transmission scheme and technology used in the common OFDM system to overcome the fading phenomenon may be roughly divided into two schemes: an Adaptive Modulation and Coding (AMC) scheme and a diversity scheme.
The AMC scheme adaptively adjusts a modulation scheme and a coding scheme according to a channel variation of a downlink. A terminal can detect Channel Quality Information (CQI) of the downlink by measuring a Signal-to-Noise Ratio (SNR) of a received signal. That is, the terminal feeds back the CQI of the downlink to a base station through an uplink. The base station estimates a channel state of the downlink using the CQI of the downlink fed back from the terminal. The base station adaptively adjusts a modulation scheme and a coding scheme according to the estimated channel state. The AMC scheme applies a higher-order modulation scheme and a higher coding rate for an improved channel state, and applies a lower-order modulation scheme and a lower coding rate for a worse channel state. Compared with the conventional scheme depending on the fast power control, the AMC scheme increases adaptability to a time-varying characteristic of the channel, thereby improving the average system performance.
The diversity technique is a scheme suitable for traffic that should not be adapted to a channel environment of a particular user, similar to the common control channel, or the traffics susceptible to delay, like the real-time traffics.
Generally, a wireless channel undergoes various changes in the time axis, and even in the frequency domain, has a good channel state in some regions and a bad channel state in other regions. In this channel environment, if it is not possible to adapt transmission data to a channel for a particular user, it is inevitable that from the viewpoint of each receiving terminal, the transmission data is received in a good channel state in some cases and received in a bad channel state in other cases. The diversity technique is suitable to be used for such environment or such traffic. The goal of the diversity technique is to allow the transmission data to evenly experience the good channels and the bad channels if possible. If particular transmission data, such as a particular data packet, is received in a bad channel state, the packet will not be demodulated successfully. Therefore, in terms of reception performance, if one packet has some modulation symbols experiencing bad channels and other modulation symbols experiencing good channels, the packet can be demodulated using the symbols experiencing the good channels.
FIG. 1 is a diagram illustrating a format of one frame in a conventional OFDM wireless communication system.
Referring to FIG. 1, the horizontal axis represents a time axis, and the vertical axis represents a frequency axis. Reference numeral 110 represents a bandwidth of the system. As illustrated, the system bandwidth 110 is composed of a plurality of sub-carriers 130. Each of the sub-carriers 130 is composed of one or a plurality of OFDM symbols in the time axis. Reference numeral 120 illustrates that one frame is composed of several OFDM symbols.
In the OFDM system, system resources are 2-dimensionally composed of a frequency domain and a time domain. That is, if the unit where one physical channel is transmitted in the time axis is defined as a frame (hereinafter also referred to as a “packet transmission interval”), the frame is commonly composed of a plurality of OFDM symbols. Since each of the OFDM symbols is composed of a plurality of sub-carriers in the frequency axis, the resource defined in one frame has a 2-dimensional resource form composed of a plurality of OFDM symbols in the time axis and a plurality of sub-carriers in the frequency axis. In the 2-dimnesional resource, the minimum unit such as one sub-carrier in one OFDM symbol, is generally called a time-frequency bin (hereinafter referred to as a “TF bin”), and the TF bin is a unit where one modulated symbol is transmitted during actual physical channel transmission.
Reference numeral 140 represents a Downlink MAP (DL MAP) transmission interval. The DL MAP 140 is used to transmit control information for user data transmitted during the frame, and has the same meaning as that of a packet data control channel. As illustrated in FIG. 1, the DL MAP 140 is transmitted in a diversity mode. In the diversity transmission mode, sub-carriers constituting a particular transmission channel are scattered over the full band. Particular positions of the sub-carriers are predefined between a base station and a terminal.
The diversity transmission mode is used to transmit the DL MAP so that a frequency diversity gain may be obtained from the diversity transmission mode. Reference numeral 150 represents a user data transmission interval where user data is transmitted in the diversity transmission mode. Reference numeral 160 represents a user data transmission interval where user data is transmitted in an AMC transmission mode. The “AMC transmission mode” refers to the transmission mode where sub-carriers constituting a particular transmission channel are localized. The AMC transmission mode means a transmission mode that can obtain a gain when a data transmitter selects a good-channel band in the frequency domain and transmits user data using the selected band. Reference numerals 161 to 166 illustrate data transmitted to different users with the AMC transmission scheme.
As described above, the conventional OFDM wireless communication system transmits the DL MAP 140, such as packet data control information, through the diversity transmission mode. The diversity transmission mode can expect performance improvement because it can obtain a diversity gain when the transmitter does not have correct information on the wireless channel environment. However, when the transmitter has correct information on the wireless channel environment, the method for transmitting packet data control information through the diversity transmission mode is not preferable in the environment where a good-channel band can be selected and allocated to each user, such as in the environment where data should be transmitted in the AMC transmission mode.
Accordingly, there is a need for an improved system and method for transmitting packet data control information separately for a diversity transmission mode and an AMC transmission mode.